The analysis will focus on the structural failure of the left wing spars. The weather was not considered to be a factor in the occurrence. The left wing structural failure may have been the result of a previous occurrence where the left wing tip had been damaged then inadequately inspected at the time of the repair. Given the leverage that existed during the wing tip strike, damage may have been done to the spar further inboard. Since ultralight aircraft are exempt from the requirement for a licensed engineer to reference a repair in an aircraft log-book, it was not possible to determine what had occurred. Normal load reversals on the spar while in service would include flight loads, landing loads, and the loads experienced while tied down. The questionable design and construction of the wing would typically result in a reduction in bending strength. Although wooden spars are susceptible to deterioration because of age and damage to a far greater degree than other spar materials, the means of actually examining these spars on the aircraft was very limited. Inspection of the wooden spar surfaces would be almost impossible without the installation of additional inspection ports. A witness reported that, during the pre-flight briefing, the pilots had discussed practising unusual attitudes and spins. Manoeuvres such as these would place higher-than-normal aerodynamic flight loads on the aircraft. The combination of factors such as previous damage, orientation of the wood grain, wing loading, and the flight profile may have exceeded the strength of the already weakened and inadequately assembled wing spars. A sudden loss of lift following the left wing spar failure would have resulted in the aircraft entering an uncommanded left roll. Recovery would not have been possible. Examination of these surfaces determined that the fracture was a result of compression damage, since the spars appeared to have been original to the aircraft, which was manufactured in 1985. The postmortem toxicology report revealed the presence of a cannabis-containing substance within the blood of the instructor; however, it could not be determined what effect the amount specified would have had on his flight performance. The following Engineering Branch report was completed:Analysis The analysis will focus on the structural failure of the left wing spars. The weather was not considered to be a factor in the occurrence. The left wing structural failure may have been the result of a previous occurrence where the left wing tip had been damaged then inadequately inspected at the time of the repair. Given the leverage that existed during the wing tip strike, damage may have been done to the spar further inboard. Since ultralight aircraft are exempt from the requirement for a licensed engineer to reference a repair in an aircraft log-book, it was not possible to determine what had occurred. Normal load reversals on the spar while in service would include flight loads, landing loads, and the loads experienced while tied down. The questionable design and construction of the wing would typically result in a reduction in bending strength. Although wooden spars are susceptible to deterioration because of age and damage to a far greater degree than other spar materials, the means of actually examining these spars on the aircraft was very limited. Inspection of the wooden spar surfaces would be almost impossible without the installation of additional inspection ports. A witness reported that, during the pre-flight briefing, the pilots had discussed practising unusual attitudes and spins. Manoeuvres such as these would place higher-than-normal aerodynamic flight loads on the aircraft. The combination of factors such as previous damage, orientation of the wood grain, wing loading, and the flight profile may have exceeded the strength of the already weakened and inadequately assembled wing spars. A sudden loss of lift following the left wing spar failure would have resulted in the aircraft entering an uncommanded left roll. Recovery would not have been possible. Examination of these surfaces determined that the fracture was a result of compression damage, since the spars appeared to have been original to the aircraft, which was manufactured in 1985. The postmortem toxicology report revealed the presence of a cannabis-containing substance within the blood of the instructor; however, it could not be determined what effect the amount specified would have had on his flight performance. The following Engineering Branch report was completed: The pilots were certified and qualified for the flight in accordance with the TC Ultralight Aeroplane Policy. The pilot lost control of the aircraft when the left wing spars failed in flight, and the aircraft entered an uncommanded left roll followed by a vertical descent into the ground. The left wing spars were not constructed in accordance with the designer's specifications. The aircraft's wing, as designed and constructed, had a questionable margin of safety in bending strength. There was evidence of previous damage to the left wing tip, which had been inadequately inspected and repaired. There were no wing inspection ports to allow for adequate periodic inspections of the internal wing structure.Findings The pilots were certified and qualified for the flight in accordance with the TC Ultralight Aeroplane Policy. The pilot lost control of the aircraft when the left wing spars failed in flight, and the aircraft entered an uncommanded left roll followed by a vertical descent into the ground. The left wing spars were not constructed in accordance with the designer's specifications. The aircraft's wing, as designed and constructed, had a questionable margin of safety in bending strength. There was evidence of previous damage to the left wing tip, which had been inadequately inspected and repaired. There were no wing inspection ports to allow for adequate periodic inspections of the internal wing structure. The in-flight structural failure of the left wing was likely caused by pre-existing damage, and by questionable design, construction, and inadequate inspection procedures. Contributing to the occurrence may have been the aircraft's gross weight and the aerodynamic flight loads placed on the wing during the training flight.Causes and Contributing Factors The in-flight structural failure of the left wing was likely caused by pre-existing damage, and by questionable design, construction, and inadequate inspection procedures. Contributing to the occurrence may have been the aircraft's gross weight and the aerodynamic flight loads placed on the wing during the training flight. Safety Action Taken Wing Construction Examination of the wreckage revealed that the aircraft had not been constructed in accordance with the manufacturer's suggestions. An article in issue 2/95 of Transport Canada's Aviation Safety Ultralight and Balloon discussed this aspect of the accident and indicated that the manufacturer was sending a related Air Safety Advisory to all known Cuby I and Cuby II owners. Wing Design In light of the identified wing design deficiencies, and the number of Magal Cuby II ultralights on the Canadian Civil Aircraft Register, a TSB Aviation Safety Advisory was sent to Transport Canada on the need to inform the ultralight community of the design shortcomings with at least some Cuby II aircraft. Ultralight Placarding The accident aircraft was not required to meet any design standards, nor was it required to be so placarded. It is not known if the student pilot was aware that the aircraft did not need to meet design standards. The draft Canadian Aviation Regulations (expected to be promulgated in 1996), include a requirement for ultralights to have a placard affixed to a surface in plain view of any occupant seated at the flight controls that states, THIS AIRCRAFT IS NOT REQUIRED TO MEET ANY AIRWORTHINESS STANDARDS/CET ARONEF N'EST PAS ASSUJETTI AUX NORMES DE NAVIGABILIT. This action should better enable occupants to manage their own risk.